A Christian group is taking the Mayor of London to court this week for violating their right to free speech by banning their posters that said homosexuals can be "reoriented" through prayer and therapy, in response to a previous ad campaign by a gay rights group.

Mayor Boris Johnson barred the Core Issues Trust from installing posters on the sides of London buses that said, "Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!" The ads were in response to a previous poster campaign by a gay rights group, Stonewall, which said, "Some people are gay. Get over it!"

Johnson believes the ads by the Christian trust are offensive to gays, and might spark retaliation against the wider Christian community, according to The Telegraph.

However, Dr. Mike Davidson, the head of the trust, and his supporters are going to the High Court on Thursday, seeking reversal of the ban on the grounds that it stifled their free speech.

"This is all about being free to talk about these issues," Davidson was quoted as saying. "It was a mistake to assume these views we were expressing came from entrenched homophobia, and failed to recognize that people who want to walk away from their homosexual feelings are a group in their own right."

His trust is going to argue that Transport for London, which comes under the mayor, has allowed ad campaigns that were offensive to Christians, most notably the one that said, "There's probably no God. Now stop worrying. And enjoy your life." These ads on London buses were funded by the prominent atheist professor Richard Dawkins and the British Humanist Association in 2009.

"The ban on these advertisements [by the Christian trust] was the beginning of a kind of reverse discrimination which threatens to obliterate debate in the public sphere," Andrea Williams, director of the Christian Legal Centre, was quoted as saying. "Boris Johnson needs to realize his mistake and ensure there is freedom for all in the marketplace of ideas. He cannot prefer one group over another."

Transport authorities have said the advertisement breached their advertising policy because it carried a "publicly controversial message" and could "cause widespread offence to members of the public."

Christians in Britain complain that authorities often take the side of those opposed to the community.

"We need to create a country in which people can be unashamedly proud of their faith – where they don't feel that they have to leave religion at the door," Baroness Sayeeda Warsi, former co-chair of the Conservative Party, earlier wrote in an article. "That means being proud of Christianity, not downgrading it."